1. Field of the Invention
The present invention relates to an ignition device that can be used for an internal combustion engine having difficult ignitability.
2. Description of Related Art
In recent years, compact and low-NOx high efficiency engines are being developed to address the demand of increase of fuel economy and reduction of CO2. High efficiency engines are difficult to ignite by sparking because they are highly supercharged and highly compressed engines which are often supplied with lean air-fuel mixture. Accordingly, there is a demand for an ignition device excellent in burning velocity and ignitability.
Japanese Patent Application Laid-open No. 2010-37949 describes a barrier discharge device for an internal combustion engine, which includes a first electrode, a second electrode surrounding the first electrode and a dielectric covering at least one of the first and second electrodes, the discharge gap between the dielectric and the other of the first and second electrodes varying in length depending on the longitudinal position of the electrodes.
However, the barrier discharge device as described in the above patent document has a problem in that the anti-inflammatory effect thereof is large causing the ignitability to be unstable, because the discharge space is formed receding radially inward greatly from the distal end of the ground electrode so that the distal end of the center dielectric is hardly exposed from the ground electrode.
Further, when a strong in-cylinder airflow is generated within a combustion chamber to promote agitation of an air-flow mixture to thereby further increase fuel economy for a lean-burn engine, if the barrier discharge device does not project to the inside of the combustion chamber at all as is the case with the above patent document, the in-cylinder airflow flows over the surface of the discharge section without reducing its speed. As a result, since a strong dragging force acts on the discharge space, radicals generated by a barrier discharge spread in the combustion chamber before they generate a flame kernel in the discharge space, preventing a volume ignition.